1. Field of the Invention
The present invention relates to a novel process for extracting useful magnesium salts from serpentine mineral wastes such as asbestos tailings. More particularly, the invention relates to a process for extracting magnesium salts from asbestos tailings, the process being characterized by substantial reduction of the amount energy and in the possibility of recycling some of the by-products formed during the extraction process.
2. Description of the Prior Art
A major by-product or waste material in the production of chrysotile asbestos is the crushed and partially ground mother rock from which the fibrous asbestos has been separated and the fines or dust of asbestos too short for any practical applications. These materials, commonly termed "tailings" and "floats" in the industry, primarily comprise hydrated magnesium silicates of the serpentine family, and typically contain magnetite and minor quantities of other minerals including iron, chromium and nickel, normally to the extent of several percent by weight, as impurities. The quantities of such waste materials or products produced daily by a modern asbestos mill amounts to thousands of tons, and over the years considerable effort has been devoted to finding a practical use or uses for these waste materials.
It is known from the prior art that strong mineral acids will readily react with or attack serpentine tailings to form corresponding magnesium salts which can then be separated from the insoluble silica residue. For example, sulfuric acid has been proposed in U.S. Pat. No. 2,402,370, June 18, 1946. Chalmers, ammonium bisulfate in U.S. Pat. No. 3,338,667, Aug. 29, 1976, Pundsack, sulfur dioxide in U.S. Pat. No. 1,865,224, June 28, 1932, Winston et al, hydrochloric acid in Canadian Pat. No. 1,034,385, July 11, 1978, Marek et al, and leaching with carbonic acid, U.S. Pat. No. 3,320,029, May 16, 1967, Adams and U.S. Pat. No. 4,058,587, Nov. 15, 1977. All these processes can generally be termed as a "wet process" since they are carried out in the presence of water and these involve complex recovery procedure of the magnesium salt formed.
The possibility of a reaction between asbestos tailings and ammonium sulfate is well proven by the patent of Dolbear (U.S. Pat. No. 1,348,933). This patent indicates that an insoluble magnesium compound such as magnesium oxide or magnesium carbonate can be reacted with ammonium sulfate at the temperature of boiling water to give the corresponding magnesium sulfate: ##EQU1##
It is obvious to anyone familiar with chemistry that since silicic acid is a much weaker acid than carbonic acid (pka of silicic acid: 2.2.times.10.sup.-10 pka of carbonic acid: 4.3.times.10.sup.-7), if the reaction works with a carbonate, it will also work with a silicate such as magnesium silicate, i.e.: tailings of asbestos.
The wet process developed by Dolbear proved to be entirely impractical on economic basis since so much water has to be distilled in order to displace ammonia that the cost of the resulting product was prohibitive. This situation arises from the fact that ammonia is very soluble in water. If some ammonia is left in water, the reaction is reversed.
However, since the dry mixture of ammonium sulfate and asbestos tailings was reacting even at room temperature I have used the dry reaction as a source of magnesium sulfate, the rate of reaction being accelerated by heating at an appropriate temperature. Of course, in using the dry mixture, the evolution of ammonia does not call for the distillation of any water, NH.sub.3 being liberated as a gas.
It is known in the chemical literature since the end of the last century that if ammonium sulfate is heated above a temperature of 250.degree. C., it will decompose into ammonia and ammonium bisulfate (Smith, J.Soc. Chem. Ind. 14, 529 (1895)) according to the following equation: EQU (NH.sub.4).sub.2 SO.sub.4 .fwdarw.NH.sub.4 HSO.sub.4 +NH.sub.3 .uparw.
Contrary to the teaching of Pundsack (U.S. Pat. No. 3,338,667) who indicates the usefulness of the bisulfate of ammonium in his process, it has been found that the decomposition of ammonium sulfate to ammonium bisulfate was very detrimental as far as the energy requirement of the reaction was concerned. In fact, in the Pundsack's disclosure, it is indicated that ammonium bisulfate rather than ammonium sulfate is the active reagent for the leaching of magnesium from the asbestos wastes either with the so-called "wet" process or "dry" process.
It has been found that the formation of bisulfate has to be limited as much as possible for the following reasons. First, the decomposition of ammonium sulfate to ammonium bisulfate is an endothermic reaction which calls for the expediture of 25.99 K cal per mole of ammonia evolved, in the standard state. Secondly, it has also been found that when ammonium bisulfate is formed, it has a marked tendency to sublimate outside the reaction mixture, thus preventing reaction with the tailings and inducing additional energy losses associated with the heat of sublimation. Repeating the reaction in a Lindberg furnace, as prescribed by Pundsack, it has been found that since the totality of the mixture quickly attains the temperature to which it is heated there is a decomposition of the ammonium sulfate into free ammonia and ammonium bisulfate, but at a temperature of about 300.degree. C. the ammonium bisulfate sublimates and has a tendency to recombine with part of the liberated ammonia thus decreasing the rate of reaction with the magnesium oxide and increasing the time and energy required to carry out the reaction to the point required to obtain a suitable yield. It is readily observed when operating this process in this fashion that a certain quantity of ammonium sulfate is to be found at the opening of the furnace thus confirming that part of the sublimated ammonium bisulfate has reacted with part of the liberated ammonia. Furthermore, it has been found that the sublimation of the bisulfate practically more than doubles the amount of energy required to achieve the reaction.
Accordingly it would appear that the advantages of operating the reaction of asbestos tailings with ammonium sulfate could be greatly enhanced if a procedure could be found whereby the ammonium bisulfate formed could be prevented from escaping from the reaction system so that the energy expanded for its preparation could remain in the system and used to advantage.